A semiconductor device having a MOS transistor and a protection diode, which are formed on a semiconductor substrate, is disclosed in, for example, EP 0905781, which corresponds to JP H11-163275. The MOS transistor is, for example, a lateral type MOS transistor, specifically, a lateral double-diffused MOS transistor (i.e., LDMOS transistor). The protection diode protects the LDMOS from an electro static discharge (i.e., ESD) or the like. In the device having the protection diode, in general, a drain of the MOS transistor is connected to a cathode of the protection diode. Further, a source of the MOS transistor is connected to an anode of the protection diode.
The withstand voltage of the MOS transistor is about 70V, for example. Further, the withstand voltage of the protection diode is about 55V, for example. Accordingly, in the device, the withstand voltage of the MOS transistor is higher than that of the protection diode by about 15V. Here, all surge current cannot be absorbed in the protection diode when the ESD is occurred.
The maximum current flows through the device, i.e., the protection diode, at a moment when the ESD is occurred. The protection diode shows a positive linear resistance characteristic after the diode is broken down. Specifically, as the current increases, the voltage of the diode also increases. This resistance region higher than 55V is attribute to a parasitic resistor in the protection diode.
When the maximum current flows through the protection diode, a certain voltage is applied to the diode. When the certain voltage is applied to the diode, the certain voltage is also applied to the MOS transistor. This is because the protection diode is connected to the MOS transistor in parallel. Accordingly, the MOS transistor may be broken down so that the transistor shows a negative resistance region. Thus, a part of the surge current flows into the MOS transistor, so that the MOS transistor is broken down.